How warped can you get?

I have an otherwise excellent copy of a John Klemmer Direct-to-Disk recording, that is warped like a potato chip about 3/8". Even with my record clamp, it is a nightmare. I can track it, but at the outermost groove, at the lead-in, it sometimes causes the stylus to leave the record slightly, but it doesn't skip. Once it gets in about 1/8" into the record, it plays well, even though the arm looks like a small boat riding the waves of the ocean in a storm. I keep this record, because it has some of the most excellent recording quality I've heard, and it is expensive to get another copy. I don't play it often, but sometimes I just get in the mood to hear it. This is the worst one I have encountered that I even tried to play.

Doug, I tried the heating the record between plates thing when I was in high school. Yes the record was flattened, and all the groove information was too. Totally non-playable after the heating. I then used it for a frisbee. It was a first pressing(when it was originally released) of Iron Butterfly "Heavy", their first album. At that time, new LPs were available for $2.99 each!

Maybe others have had better results, but that episode cured me from bringing any other records near the oven!

It should go in the phono stage, and many phono stages have them built in. However as any filter network, it has a "knee" with corresponding rolloffs. If the filter knee is set high enough to cover everything below 20Hz, it will have an effect up into the areas above 40Hz. This can compromise bass performance, since it is being partially filtered out. If the knee is set down below 10Hz, it may not be fully effective at some of the frequencies you want it to filter. It requires an educated design, based upon experience. There are compromises in everything.